anv: align the batch buffer length to 8 to meet kernel request

Align the length of batch buffer to 8. Fix the invalid parameter error
when call ioctl to send batch buffer to kernel.

Kernel will call i915_gem_check_execbuffer to check the batch buffer
length and batch_start_offset.

Signed-off-by: Yang Shi <yang.a.shi@intel.com>
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